[PATCH 0/6] getenv() timing fixes

Similar to the recent:

  https://public-inbox.org/git/20190109221007.21624-1-kgybels@infogroep.be/

there are some other places where we do not follow the POSIX rule that getenv()'s return value may be invalidated by other calls to getenv() or setenv().

For the most part we haven't noticed because:

  - on many platforms, you can call getenv() as many times as you want.
    This changed recently in our mingw_getenv() helper, which is why
    people are noticing now.

  - calling setenv() in between _often_ works, but it depends on whether
    libc feels like it needs to reallocate memory. Which is itself
    platform specific, and even on a single platform may depend on
    things like how many environment variables you have set.

The first patch here is a problem somebody actually found in the wild. That led me to start looking through the results of:

  git grep '= getenv('

There are a ton of hits. I poked at the first 20 or so. A lot of them are fine, as they do something like this:

  rla = getenv("GIT_REFLOG_ACTION");
  strbuf_addstr("blah blah %s", rla);

That's not _strictly_ correct, because strbuf_addstr() may actually look at the environment. But it works for our mingw_getenv() case, because there we use a rotating series of buffers. So as long as it doesn't look at 30 environment variables, we're fine. And many calls fall into that bucket (a more complicated one is get_ssh_command(), which runs a fair bit of code while holding the pointer, but ultimately probably has a small fixed number of opportunities to call getenv(). What is more worrisome is code that holds a pointer across an arbitrary number of calls (like once per diff'd file, or once per submodule, etc).

Of course it's possible for some platform libc to use a single buffer. But in that case, I'd argue that the path of least resistance is wrapping getenv, like I described in:

  https://public-inbox.org/git/20181025062037.GC11460@sigill.intra.peff.net/

So anyway. Here are a handful of what seem like pretty low-hanging fruit. Beyond the first one, I'm not sure if they're triggerable, but they're easy to fix. There are 100+ grep matches that I _didn't_ audit, so this is by no means a complete fix. I was mostly trying to get a sense of how painful these fixes would be.

  [1/6]: get_super_prefix(): copy getenv() result
  [2/6]: commit: copy saved getenv() result
  [3/6]: config: make a copy of $GIT_CONFIG string
  [4/6]: init: make a copy of $GIT_DIR string
  [5/6]: merge-recursive: copy $GITHEAD strings
  [6/6]: builtin_diff(): read $GIT_DIFF_OPTS closer to use

 builtin/commit.c          |  3 ++-
 builtin/config.c          |  2 +-
 builtin/init-db.c         |  6 ++++--
 builtin/merge-recursive.c | 15 ++++++++++-----
 diff.c                    |  5 ++++-
 environment.c             |  4 ++--
 6 files changed, 23 insertions(+), 12 deletions(-)

-Peff